Detection of toxic gases



Aug. 5, 1947. w. F. FAGEN DETECTION 0F TOXIC GASES Filed March 6;, 1943 Patented Aug. 5, 1947 DETECTION F TOXIC GASES William F. Faxen, Chicago, Ill., assigner to Stewart-Warner Corporation, Chicago, Ill., a corporation of Virginia Application March 6, 1943, Serial No, 478,323 17 Claims. (Cl. Z50-41.5)

The present invention relates to improvements in the detection of toxic gases and particularly' to improved apparatus of the type including bridgev circuit arrangements comprising photosensitive devices.

In the contemporaneously filed application of Willard H. Farr, myself, and Stanley Kolanowski, Serial No, 478,324, March 6, 1943, there is disclosed and claimed apparatus for detecting toxic gases, particularly carbon monoxide. The appa.- ratus disclosed includes a differential bridge circuit having in it a pair of photosensitive devices. The bridge is energized from a battery and the control circuit associated with the bridge, which includes-an electron tube, is energized from a low voltage source, such as the 24 volt source ordinarily found in airplanes for which the apparatus was particularly designed.

The present invention has for its primary object the provision of a new and improved control circuit whereby the apparatus may be energized from a single low voltage supply source, thereby eliminating the use of batteries or other ancillary power supply circuits.

A further object of the present invention is the provision of new and improved photosensitive gas detectionv apparatus operable from a single low voltage supply source and having means for compensating for like variations in the amount of light striking the photoelectric devices.

Another object of the present invention is to provide an improved differential photosensitive circuit arrangement including electronic tubes controlled by the photosensitive devices, wherein an indication is given of inoperative conditions of the electronic tubes, for instance, when one or the other of the tubes is burned out.

Other objects and advantages of the present invention will become apparent from the -ensuing description, in the course of which reference is had to the accompanying drawings, in which Fig. 1 is a diagrammatic representation of one embodiment of the invention; and

Fig. 2 is a view similar to Fig. 1, of a second embodiment of the invention;

Before proceeding with a detailed description of the present invention, it may be well briefly to consider the primary use of the circuit arrangement of the present invention, which, as indicated above, is the detection of carbon monoxide, and particularly small quantities in short time intervals. a pair of photosensitive devices are utilized. These are connected in a bridge circuit and supplied with light from a single light source, preferably located between the devices. Associated with the devices is a substance having an photocharacteristic-preferably the light transmission characteristic-that changes in the presence of carbon monoxide. This substance is preferably placed in glass tubes, each of which is located between the light source and an associated one of the photosensitive devices. Fresh air is passed through one of the tubes, which tube acts as a standard, while the air to be tested for the presence of carbon monoxide is passed through the second tube. The photosensitive devices are preferably connected in a differential balanced bridge circuit so that any unbalance of the bridge,as al result of the change in color transmitting characteristics of the substance in the tube through which the air to be tested is passed, is utilized to control electron tubes controlling some device such a warning light, or the like.

The present apparatus is intended, primarily,

is preferably either fresh air from outside the cabin, or the fresh air in the cold air duct leading to the heat exchanger. The air'to be tested is the heated air taken either from the inside of the cabin or from the hot air duct through which the heated air is -supplied to the interior of the cabin.

Referring now to Fig. 1, the photoelectric tubes, which may be of the 923 type, are indicated by reference characters l0 and I2. These are located a short distance apart and between them is a light source, such as the lamp i4, which is continuously energized when the apparatus is in operation. The lamp is mounted within a housing I8, which also houses a pair of glass tubes I 8 and 20 which .contain a substance the color of which changes in the presence of carbon monoxide from a yellow progressively to a dark, blue-green. This photo-.characteristic change is accompanied, among other things, in the change in the amount of light tranmitted therethrough and this change is utilized in the apparatus to actuate a relay or the like. The carbon monoxide sensitive substance is described in the aforemen- In the detection of carbon monoxide tioned contemporaneously led application and the applications referred to therein.

The air to be tested, in this. case the heated air, is supplied to glass tube I8 through a conduit 22 and the fresh air is supplied to the tube 20 through a conduit 24. It is preferred that the air passing through the tubes, that is, both the heated and fresh air, be at approximately the same temperature and that the velocity be relatively low. The apparatus for bringing them to the same temperature, restricting the Ilow, and the physical construction of the detection apparatus are more fully ldisclosed and also claimed in the Ipreviously referred to application Serial across a pair of low voltage supply lines 28 and 28. which may have a potential difference of 24 volts, the voltage usually used in airplanes. Photoelectric tube I is connected across the supply lines through a resistor 30, the tube being connected to the positive line 28 and the resistor to the negative line 28. The photoelectric tube i2 is similarly connected across the supply lines through an adjustable resistor 32. The resistor 36 may have a resistance of about three megohms and the resistor 32 a. resistance of about five megohms.

The photoelectric tubes are utilized to control a pair of electron tubes 34 and 38 of the tetrode type (such, for instance, as the type 25B6G) and these tubes in turn control the operation oi' a differential relay 38. The relay has opposed windings 40 and 42 in the plate circuits of tubes 34 and 36, respectively, and a holding coil 44, which is effective under balanced conditions to maintain in closed position the relay operated switch 46. Switch 46 is either gravity or spring biased into open position.

The control grids 48 and 56 of electron tubes 34 and 36, respectively, are connected to the junctions of tube I2 and resistor 32 and of tube I6 and resistor 36 through conductors 82 and 84, respectively. The arrangement is such therefore that, as the amount of light impinging upon the two tubes is varied, the grid potentials of the tubes change to vary the flow of current through the differential windings 40 and 42 of the relay.

vIn order to compensate for like variations in the amount of light transmitted to the photoelectric tubes and thus to compensate for aging of the carbon monoxide responsive substance or the like, the grid connections are identically connected to their respective photoelectric tube circuits. In the instant embodiment the arrangement is such that when less light falls upon the photoelectric tubes, the grids become more negative as a result of the decreased flow of current through the resistors 30 and 32. Under ordinary or normal conditions the grids of the electron tubes are maintained at equal potentials so that the differential windings 40 and 42 are energized equally. This condition may be attained by adjustment of resistor 32 in circuit with photoelectric tube i2. When, however, carbon monoxide is present in the heated air stream passing through tube i8, less light falls upon photoelectric tube I0 than upon tube I2, with the result that the grid 50 of tube 36 becomes more negative than the grid 48 of tube 34, and the tube 36 thus passes less current than tube 34. Winding 42 of the differential relay is thus less effective than Winding 40 and, at some current value determined by a sensitivity adjustment to be described hereinafter, the lunbalance results in the opening of switch 46. It may be seen, therefore, that the winding 42 is so arranged as to tend to hold the switch closed, as is the holding winding 44. The winding 40, on the other hand, tends to open the switch.

Relay winding 40 is connected to the anode 56 of the tube 34 by a conductor 51 and to the positive supply line by a conductor 58. In similar manner relay winding 42 is connected to the anode 6U of tube 36 by conductor 62 and to the positive line 26 by the conductor 56.

The relay operated switch 46 is held in closed position under balanced current conditions by the winding 44, one terminal of which is connected to the power supply line by conductor 64 and the other to the negative supply line 28 by a conductor 66, the cathode heater 66 of tube 34, and a conductor 10.

The electron tube and relay arrangement is such that the switch 46 opens in the event either of the tubes burns out. This is accomplished by connecting the relay winding 44 in series with the cathode heater 68 of tube 34 and by making the holding winding 44 stronger than either of windings 40 and 42 and just strong enough to hold the switch 46 in its operated or closed position under balanced conditions. Thus, under balanced conditions the switch is held closed by winding 44 but, should the tube 34 burn out, windings 44 and 40 will become ineffective, leaving effective only winding 42. This winding, however, is not strong enough to hold the switch 46 in its closed positionso that when tube 34 burns out, the switch 46 is returned to its open position. In the event tube 36 should burn out, then winding 42 becomes ineffective, leaving the opposing windings 40 and 44 in circuit. The resultant effect is too small to hold the switch closed.

'I'he cathode heater 12 of tube 36 is connected directly across the power supply lines by conductor 14 and the previously mentioned conductor 16.

The sensitivity of the apparatus is controlled by varying the portion of grid bias resistor 18 in the cathode circuits of the electron tubes. This is done by an adjustable resistor contact 18 connected by a conductor to the cathodes 82 circuits and momentarily closing a normally' open push button switch in circuit with relay winding 42. The switch is connected across conductors 62 and 26 through a current limiting resistor 92. This arrangement enables the relay winding 42 readily to be energized to an extent suflicient to move the switch 46 to its closed position.

The relay operated switch 46 may be utilized to control a signal lamp or to operate some device controlling either the supply of air to the airplane cabin or the operation of the heater. The control can be effected through the circuit 94 in which the switch 46 is located. When the switch is closed, the circuit 94 is also closed and when the switch is open, the circuit is opened.

In operation, the power supply lines 26 and 28 are connected to a low voltage source. of current through the tubes and through the differential windings 40 and 42 is adjusted by adjustment of resistor 32 until the current flow is equal. As a result, the windings exert equal and opposing forces upon the switch 46 whereas the holding winding 44, which is supplied with the same current during normal operation, tends to hold the switch 46 in its closed position. To close switch 46 initially, it is necessary only momentarily to close switch 96, whereupon winding 42 is excessively energized and assists winding 44 to close the switch. Thereafter, the switch is maintained in its closed position by winding 44.

As long as there is no carbon monoxide in the heated air flowing through tube i8 and the iight The flowA leek so that carbon monoxide from the exhaust geses is introduced into the heated sir stream, then the carbon monoxide changes the sensitive gel from its original yellow color progressively to s duri: blue-green color. Less light, therefore. ,casses to photoelectric tube I and the grid 60 oi tube 3e becomes more negative, as heretofore expleined. Less current flows through tube I6 and through relay winding 42, until the current reaches some low value at which winding l0 overcomes the joint eiIect of windings t! und u end switch 46 opens. When the switch opens, the control circuit 94 is likewise opened for the purm ci' (l) indicating the presence of the carbon monoxide, (2l operating some device controlling the operationl oi' the heat exchanger or the supply of heated cir to the cabin, or (3) both indicsting and opereting some device., vline value of current at which the relay opens is determined by the sensitivity adjustment consisting ol the adjustable Contact 'i8 ond the grid bles resistor it, which, as explained heretofore, changes the grid bias applied to both tubes.

Once the relay has been operated then, lo: the apparatus again to be operated to detect and signal the presence of carbon monoxide, o new tube le oi sensitive gel muy be used or the resistor 32 muy be adjusted to rebalance the circuit. The letter operation, of course. can be carried on only if the sensitive gel has not reached e darkened color such that it is no longer sutil-l ciently sensitive to the presence of carbon momozride. However, if it has not become too ineen sitive, then thecircuit may be rebalanced by adjustment oi resistor 32 and the switch l again momentarily closed to reset the relay Thereeftezg'if carbon monoxide is again detected, the apparatus oberstes in the manner described obove to release the relay..

It is advantageous to connect the photoelectric tubes lil ond i2 to the positive power supply line end the resistors to the negative power supply conductor. The reason lor this is that the tubes ore operated at e. higher potential, and, when less light impinges upon them, their cathodes become more negative as do the grids of the elecEl tron tubes, which are connected to the cothodes. Thus, if the iight source 'M associated with the photoelectric tubes should burn out, the grids become most negative and reduce the dow oi current through the tubes. If the circuit was arronged to make the grids more positive when less light impinged on the photoelectric tubes, then, in the event the lamp burned out, the now ci current through the tubes would be increased. This latter result is not at all desirable.

il somewhat different circuit arrangement is illustrated in the embodiment of Fig. 2. Here o ngle winding relay is used and the flow of our ent through it is controlled by e. pour of cleo tron tubes. These ere so arranged that when cerbon monoxide is present the totsl ow ci ourn rent through winding is reduced to release the reloy. iolripensetion for like verietions in e sensitive substance, es by darkening result= i g from aging, is provided by errenging the so that the iiow of current through one decreases und that through 'the other in bugia conductive.

crosses, the totei dow remaining substantially constant.

Referring now to lfig. 2 the photoelectrlc tubes it@ and let ore connected across the power supply lines 2li und 2li through the adjustable resisters 506 and tilt, respectively. The tube Still is connected to the positive terminul, whereas the tube ili2 is connected to the negative terminal, so that when less light lmpinges upon the tubes,'the more negetive becomes the cathode of the tube S00, whereas the snode of tube W2 becomes more positive.

ln the embodiment of Fig. 2 the effective tube is tube itil, i. e., lt is the tube associated with the heated oir supply. which is likely to become contaminated with carbon monoxide.

The electron tubes controlled by the photoelec- -tric tubes ore indicated by the reference cher-u acters 4% und lill, respectively. The control grid ila oi the ion/ner is connected to the cathode of photoelectrlc tube iilii by o conductor iid end the control grid lid ol the tube lib is connested to theanode or photoelectric tube W2 by e. conductor lill. l

The snode circuits or the two electron tubes are connected in parallel and the current flowing through the two tubes flows through the relay winding llt. @ne terminal of the relay winding is connected to the positive power supply line 2B by conductor lll und the other terminal is connected to the anode ile of tube le@ by conductors t28 end t28. This other terminal is also connected to the anode lil@ of tube lill by conductors t28 end i32. The cathode heaters i554 ol the two tubes are connected in parallel and across the power supply lines by conductors i3d and 133. The cathodes im and lei of the two tubes are joined together and connected by s. conductor its to an adjustable contact @te which serves to com trol the portion of a grid bias resistor iBS in the cathode circuit. One terminal of the bias resistor is connected to the negative supply line 2o by a conductor ii und the other terminal is connected to the positive line 26 through bleeder resistor 52.

The relay operates s. switch 55d which is normolly closed, i. e., when the apparatus is in operation and in balance. ri'he switch controls a circuit ice controlling e. signal lomp or some device lor controlling the heated air to the cabin, or the like.

' After the apparatus has once operated, it may be reset after balanced conditions obtain by momentsry closure of a manually opereble resetting switch E58, which energlzes winding l2@ through e current limiting resistor idd.

When the apparatus is initially placed in op-n eration, the resistors @EM and lilo in circuit with the photoelectric cells ore adjusted until the tubes E08 and 'lti poss approximately the same current. The .sensitivity control ldd is then adjusted to somenoint et which the totul current howing through the two tubes and through winding i2@ is cumulent to meiutuin the switch |54 closed upon the momentary closure o the resetting switch idd.

When no carbon monoxide is present in the sir, then the beisnced condition described obove obtains and the relay remains in its operated position. In the event the sensitive gel in the tubes it und (which have been omitted from Fig. 2) derlsens es e result oi? engine or the like, then the photoeiectric tubes lill@ end tilt become less The grid lill or" electron tube lilii rendered more negative, whereas the grid iiii of electron tube lli! becomes more positive and ilow o! current through tube |08 is decreased. whereas the ilow of current through tube Iii is increased. The changes in now oi' current through the two tubes approximately balance so that there is but little variation in current ilow through the relay winding ineanfd' the switch IN remains in its operated position.

When carbon monoxide is present in the heated air, then photoelectric tube IM is illuminated less than tube |02 and the flow oi current through electron tube |08 isV decreased. This decrease in current ilow through the tube results in a decrease in current ilow through the relay winding and, at some current value dependent upon the sensitivity adjustment, the switch Ill is opened. The switch Ill may, like the previously described switch 4l, be biased into its open position by gravity or a spring.

After th'e device has operated as above indicated, it may be reset for operation, provided the color ot the sensitive gel has not changed excessively, by readjusting the photoelectric cell circuits and momentarily closing the reset switch IBI.

While the present invention has been described as being particularly adapted for use in connection with low voltage supply circuits, it should be understood that certain features thereof may be used in conjunction with higher voltage sources. In addition. the circuit arrangement may be advantageously utilized for the detection of substances or materials other than carbon monoxide, although it has proven to be extremely advantageous in the detection of carbon monoxide.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent of the United States is:

1. In photosensitive detection apparatus, the combination including, a pair oi photosensitive devices for providing two control voltages in response to the passage thereby of the substance to be detected, a pair of electron tubes o! the type having control electrodes, said electrodes being supplied with said control voltages. a single relay controlled by both said tubes, and means associating said relay, tubes, and photosensitive devices so that said relay is normally in operated position and movable into non-operated position by dissimilar changes in ligh't striking said photosensitive devices.

2. In photosensitive detection apparatus, the combination including, a pair oi' photosensitive devices for providing two control voltages. both o! which become more negative as less light strikes the devices,` a pair of normally conductive electron tubes of the type having control electrodes, said electrodes being supplied with said control voltages, and a diil'erential relay normally held in operated position controlled by both said tubes, said relay including a pair of opposed windings i each of which is in the anode circuit of one o! said tubes. y

3. In photosensitive detection apparatus, the combination including, a pair of photosensitive devices for providing two control voltages, both of which vary in the same manner in response to like changes in the amount of light striking the devices, a pair of electron tubes of the type having control electrodes, anodes, and cathodes, said electrodes being supplied with said control voltages, a source of anode potential for said tubes, a cathode resistor adjustably connected in circuit with the cathodes of both said tubes connected across said source, and a differential relay controlled by both said tubes.

4. In photosensitive detection apparatus, the combination including, a pair of ph'otosensitive devices for providing two control voltages, both v of which vary in the same manner in response to like changes in the amount of light striking the devices, a pair ot electron tubes of the type having control electrodes, said electrodes being supplied with said control voltages, a differential relay including opposed windings controlled by both said tubes and operable into one position upon a predetermined change in light striking the devices, a source of anode potential, said windings and tubes being connected across said source, and means including a switch for connecting one of said windings to said source independently o! said tubes to operate said relay into another position.

5. In photosensitive detection apparatus, .the combination including, a relatively low voltage supply source having positive and negative lines, a pair oi' photosensitive devices each connected across said lines through associated resistors, said resistors being connected to the negative line, a pair of electron tubes of the type having control electrodes, anodes, and cathodes, connections from each electrode to the junction of one of said devices and its associated resistor, a diiferential relay normally held in operated position controlled by both said tubes, said relay including a pair oi' opposed windings each of which is connected across the anode of one tube and the positive line, resistance means connected across said lines, and an adjustable connection from the cathodes of said tubes to said resistance means.

6. In photosensitive detection apparatus, the combination including, a pair of photosensitive devices for providing two control voltages, both' of which vary in the same manner in response to like changes in the amount of light striking the devices. a pair of electron tubes oi the type having control electrodes and cathode heaters, said electrodes being supplied with said control voltages, and means including a differential relay having a holding winding in series with the cathode heater of one tube for indicating operativeness ot the apparatus.

7. In photosensitive detection apparatus, th'e combination including, a pair of photosensitive devices for providing two control voltages, both I oi.' which vary in the same manner in response to like changes in the amount of light striking the devices, a pair of electron tubes of the type having control electrodes and cathode heaters, said electrodes being supplied with said control voltages, and a relay controlled by both said tubes, said relay including a pair of opposed windings connected one in the anode circuit of each tube and a third winding aiding one of said pair of windings connected in series with th'e cathode heater of one tube for holding said relay in operated position under balanced current fiowythrough said pair of windings.

8. In photosensitive detection apparatus, the combination including, a relatively low voltage supply source having positive and negative lines,

a rst and second photosensitive device connect-` ed across said lines, means for decreasing the light striking one of said devices in response to the presence of the material to be detected, resistors interposed between said devices and the negative lines, a rst and second electron tube of the type having control electrodes, anodes, cathodes, and cathode heaters, connections from the electrodes of said rst and second tubes to the junctions of said rst and second devices in their associated resistorsrespectively, a relay normally held in operated position controlled by both said tubes, said relay including a pair or opposed windings, the first of which is connected across the anode of said ilrst tube and the positive line, and the second of which is connected across the anode of said second tube and the positive line, said relay including also a third winding aiding the iirst and a switch movable into circuit closing direction by said irst and third windings, said first two windings being unable to hold said switch closed and said third being unable to close it but able to hold it closed,

.and means connecting said third winding in series with the cathode heater of said second tube.

9. In photosensitive detection apparatus, the combination including, a pair of photosensitive devices for providing two control voltages varying in opposite senses in response to like changes in light striking said devices, a pair of electron tubes of the type having control electrodes, said electrodes being supplied with said control voltages, and a relay having a single winding connected in series in the anode circuits of both said tubes.

10. In photosensitive detection apparatus, the combination including, a pair of photosensitive devices for providing two control voltages one of which becomes more negative and the other more positive in response to less light striking said devices, a pair of electron tubes of the type having control electrodes, said electrodes being supplied with said control voltages and the anode circuits being connected in parallel, and a relay having a single Winding in the Parallel connected anode circuits of said tubes and normally closed switch means adapted to be opened when less light strikes said one oi" the devices than the other.

11. In photosensitive detection apparatus, the combination including, a pair of photosensitive devices for providing two control voltages varying in opposite senses in response to like changes in light striking said devices, a pair of electron tubes of the type having anodes, cathodes, and

control electrodes, said electrodes being supplied with said control voltages and said anode-cathode circuits being connected in parallel, a cathode bias resistor in the cathode circuit of both said tubes, and a relay having a single winding in the parallel connected anode circuits of both said tubes.

12. In photosensitive detection apparatus, the combination including, a pair of photosensitive devices for providing two control voltages varying in opposite senses in response to like changes in light striking said devices, a pair of electron tubes of the type having control electrodes, said electrodes being supplied with said control voltages, a relay having a, single winding in the anode circuits of both said tubes, and switch means for changing the current iiow through said relay independently of said tubes.

13. In photosensitive detection apparatus, the combination, including, a relatively low voltage supply source having positive and negative lines, a pair of photosensitive devices each connected across said lines through associated resistors, one

y of said resistors being connected to the negative line and the other to the positive line, a pair of electron tubes of the type having control electrodes, anodes, and cathodes, connections from each electrode to the junction of one of said devices and its associated resistor, a relay normally held in operated position controlled by both said tubes, said relay including a single winding connected across the anodes of both tubes and the positive line, resistance means connected across said lines, and an adjustable connection from the cathodes of both said tubes to said resistance means.

14. In photosensitive detection apparatus, the combination including, a control circuit including a pair of photosensitive devices, a pair of electron tubes controlled conjointly by said control circuit, a relay controlled by both said elecl' means associating said relay, tubes, and control circuit so that a change in current ratio occurs and said relay is operated by dissimilar changes in light striking said photosensitive devices.

16. In photosensitive detection apparatus, the combination including, a control'circuit including a pair of photosensitive devices and a pair of normally conductive electron tubes controlled thereby, a relay normally energized and operated by the current flowing through said electron tubes, and signal means operated by said relay in its energized position, whereby said signal means is effective also to provide a signal of the inoperativeness of the apparatus.

17. In photosensitive detection apparatus, the combination including, a control circuit including a pair of photosensitive devices, a pair of electron tubes of the type having control electrodes, anodes and cathodes, said electrodes being supplied .with control voltages from said control circuit, a source of potential, said anodes-and photosensitive devices being connected t0 said source, a cathode resistor adjustably connected in circuit with the cathodes oi. both said tubes and `connected across said source, and a relay con- REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,999,646 Wittkihns Apr. 30, 1935 2,063,765 Smola Dec. 8, 1936 2,070,178 Pottenger, Jr., et .al Feb. 9, 1937 2,100,934 Berges Nov. 30, 1937 2,085,010 Dillon June 29, 1937 2,140,355 Gulliksen Dec. 13, 1938` -1,936,400 Langmuir Nov. 21, 1933 2,238,762 Whitaker Apr. 15, 1941 2,237,665 Gulliksen Apr. 8, 1941 1,890,640 Deisch Dec. 13, 1932 2,173,164 Hansell Sept. 19, 1939 FOREIGN PATENTS Number Country Date 47,914 France Aug. 23, 1937 y 788,111 France Oct. 4, 1935 Certificate of Correction Patent No. 2,425,059. 4 August 5, 1947. WILLIAM F. FAGEN It is hereby certified that error appears in the printed speeication of the above numbered patent requiring correction as follows: Column 10, line 12, claim 14, strike out the Word oonjontly and insert the seme after controlled in line 13, same claim; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 14th day of October, A. D. 1947.

THOMAS F. MURPHY,

Assistant Uommzssz'oner of Patents. 

